Method and apparatus for reproducing magnetically recorded signals



March 12, 1957 A. A. TUART, JR

JNVENTOR.

% ALFRED A. STUART JR.

Unite States Patent O METHOD AND APPARATUS FOR REPRODUCENG MAGNETICALLY RECORDED SIGNALS Alfred A. tuart, Jr., Hasbronck Heights, N. 1., assignor to Bendix Aviation Corporation, Teterboro, N. 5., a corporation of Delaware Application October 20, 1949, Serial No. 122,542 11 Claims. (Cl. 179-1902) The invention relates to a method and apparatus for reproducing a signal recorded magnetically on a ribbon, wire or tape of magnetic material.

The terms magnetic record, ribbon-like element, ribbon, wire and tape are used throughout the specification and claims to indicate an elongated element of permeable material on which a signal may be recorded magnetically. The ribbon may comprise an element of rectangular cross-section on which the lines of magnetic force are disposed substantially transversely of the element, or the ribbon may comprise an element of circular cross-section in which the lines of magnetic force are disposed lengthwise of the element.

In apparatus of this kind used heretofore, signals resulting from speech, music or the like, are recorded magnetically on the ribbon by moving the ribbon at a pre determined speed through the field of an electromagnet varied in accordance with the signals. The signals are reproduced or played back by moving the magnetized ribbon at the predetermined speed through a pickup coil similar to the recording electromagnet. Voltages are induced in the coil by variations in the strength of the magnetic field as the ribbon moves through the coil. With this arrangement, the amplitude of the induced voltage is determined by the degree of magnetization of the ribbon, by the speed of the ribbon, and by the frequency of the recorded signals.

In another arrangement, shown in Patent No. 2,378,388, issued June 19, 1945, to S. I. Begun, a carrier current is modulated by the signals and the resultant is recorded magnetically on the ribbon. Ordinary electromagnetic recording and reproducing heads are used to record and play back the signal. The amplitude of the play-back voltage is determined by the degree of magnetization of the ribbon, by the speed of the ribbon and by the frequency of the carrier current.

With each of these arrangements, the ribbon must move relative to the electromagnet at a substantially rapid rate when playing back the signal or the amplitude of the induced voltage during play-back is too small to reproduce the signal, especially when the recorded signal is of low frequency,-such as in recording bass tones.

One object of the present invention is to provide a novel method of and an apparatus for reproducing signals in which the amplitude of the reproduced signal voltage is independent of ribbon speed.

Another object is to provide a novel method of and an apparatus for reproducing signals in which signal recordings currently in use may be accurately reproduced.

Another object is to provide a novel method of and an apparatus of high fidelity for reproducing signals of a wide range of frequencies.

Another object is to play back signals on a ribbon of magnetic material or any suitable form at a slower speed than heretofore and yet accurately reproduce the signal.

The invention contemplates a pole piece having an exciting coil and a pickup coil closely associated with ice a ribbon-like element on which the signals are recorded. The exciting coil preferably is energized by a saturating current having a frequency in excess of the recorded frequency, and as the ribbon-like element moves relative to the pole piece, a voltage corresponding to the recorded signal is induced in the pickup coil.

The foregoing and other objects and advantages of the invention Will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawing wherein one embodiment of the invention is illustrated. It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only, and is not to be construed as defining the limits of the invention.

In the drawing,

Figure 1 is a schematic view showing the mechanical arrangement of a recording-reproducing system incorporating novel reproducing apparatus according to the present invention, and

Figure 2 is a detailed schematic diagram illustrating one form of reproducing apparatus according to the present invention.

In Figure 1, an endless ribbon, wire or tape 1 of suitable magnetic material is driven about rollers 3, 5 by an electric motor 7 through a belt 9. Instead of using an endless ribbon and rollers, a ribbon of any desired length may be wound from one spool to another. A recording electromaguet 11 of any suitable kind is closely associated with the ribbon and is connected to input terminals 13, 15 to which the signal is applied. The above elements are well known in the art and require no detailed description.

A novel magnetic pickup, constructed according to the present invention and shown in detail in Figure 2, ineludes a core member 16 of highly permeable material, such as Permalloy or Mumetal, having a pair of pole pieces 17, 19 with a small air gap 20 therebetween through which the magnetized ribbon passes. Each pole piece has an aperture 59 extending transversely therethrough and a pair of exciting coils 21 are wrapped on each pole piece adjacent the aperture. The coils are connected in series with a secondary 25 of a transformer 27 having its primary 29 energized by a high frequency source 23 preferably from 25,000 to 75,000 cycles per second. Any desired frequency may be used, but frequencies considerably higher than the signal frequency are preferred. The pole pieces and ribbon preferably are positioned so that the lines of magnetic force of their respective fields extend parallel to one another.

An undulating exciting current is preferred to an alternating current because with undulating current, higher frequencies may be used more effectively than with an alternating current. For this reason, a direct current source, such as a battery 23, is connected in series with exciting coil-s 21 and secondary 25. The characteristics of the permeable core member are such that its power output attains a maximum at a relatively low frequency when an alternating exciting current is used, and thereafter, any increase in the frequency of the alternating.

current results in a gradual decrease in the power output, whereas with an undulating current, the power output increases even with relatively high frequencies.

A pair of pickoif coils 31, 32 are wound on pole pieces 17, 19, respectively, and the pickoif coils are connected in series with one another. One terminal of coil 31 on pole piece 17 is connected to the grid 33 of an amplifier tube 35, and one terminal of pickoff coil 32 on pole piece 19 is connected to the cathode 37 of tube 35. Plate 39 of tube is connected in series with the primary 41 I of a transformer 43 and to a 3+ voltage supply. The secondary 45 of transformer 43 may be connected to a de- 17 19, and no voltage is induced in coils31, 32. When 'pole pieces 17,19 are periodically saturated by alternating or undulating current supplied to exciting coils .21,

then the reluctance of the path taken by theribbon field in pole pieces 17, 19 increases periodically and forces the ribbon field to leave pole pieces 17, 19 and flow through a path of lower reluctance in the air at a rate determined by the frequency of the alternating or undulating current. 32 which has the same frequency as the exciting frequency when undulating current is used and double the exciting frequency when alternating current is used. When the ribbon moves relative to the pickup, and the ribbon field varies along its length, then the amplitude of the induced voltage is modulated in the same way as a modulated radio frequency carrier. When the ribbon is at rest relative to the pickup, a'n unmodulated voltage results. The amplitude of the voltage is determined only by the strength of theribbon field and notby ribbon'speed. 7

' Coils 31 and 32 are arranged on pole pieces 17, 19 so that no'voltage is induced in the coils due to staturation of pole. pieces i7; 19 in the absence of a ribbon field.

The field caused by saturation of 'pole' pieces 17, 19

flows in part about apertures 59, as indicated. by the dotted lines in Figure 2, and since the lines of magnetic force cross the windings of coils 31, 32 equally in opposite directions, no voltage is induced in the coils.

The arrangement described permits the use of signal frequencies down to zero frequency, and is especially desirable in reproducing bass tones with high fidelity. Also, the-amplitudes of the reproduced signals are independent of ribbon speed, and the ribbon, when recording the signals, may be moved relative to the recording head at any desired speed, bearing in mind that a shorter length of ribbon will be required'for a given series of si alsat low ribbon speeds than at higher ribbon speeds.

Also, the device may be used to play back signals already recorded magnetically on a ribbon in the usual manner.

Although but one embodiment of the invention has been illustrated and described in detail, it is to'be expressly'understood that the invention is not limited thereto. For example, pole pieces 17, 33 may be formed separatelyor only a sin le pole piece need be used, but

'the arrangement shown and described provides an inable member for said pick-up coil adapted for relative movement with respect to the record in close proximity thereto so that said member receives'fiux from the record, and means for periodically saturating said mem her when said member receives flux from the record to induce voltages in-said pick-up coil corresponding to the signals. 7 I p r I 2. In a device for translating a magnetic field recorded Unit 47 may include a rectifier This induces an alternating voltage in coils '31,

on a magnetic record, pick-up winding means, a structure including a magnetically permeable portion for said pick-up winding means adapted for relative movement with respect to the record so that said portion receives flux from the record, and means for periodically saturating said portion at a frequency higher than the signal frequencies to vary periodically the reluctance of said portion. V

3. A method for'translating a magnetic field recorded on a magnetic record with the aid of a pick-upstr-ucture having a magnetically permeable portion which corn prises the steps of introducing flux from the record into the magnetcally permeable portion while periodically varying the permeability of the magnetically permeable portion by periodically:substantially saturating said portion.

4. A method for reproducing signals from a magnetic record with the aid of a pick-up structure provided with pick-up winding means and having a magneticallyperme able portion for the pick-up winding means, which com prises introducing unidirectional flux from the record into said magneticallypermeable portion and simultarecorded signals.

5. A method for converting a magnetic field recorded on a magnetic record into electrical signals with the aid of a pick-up device having pick-up winding means and two magnetically permeable portions for said wind: ing means, which comprises the steps or" introducing'fiux from the record into said magnetically permeable portions at the signal frequencies of the recorded magnetic. field while periodically at a frequency higher than the highest signal frequency introducing'additional flux into said magnetically permeableportions so that the efiective flux in said portions is unbalanced to induce in the pickup winding means a voltage corresponding :to the recorded magnetic field.

6. In a device for reproducing signals from a magnetic record, pick-up winding means, a magnetically permeable member for the pick-up winding means adapted for relative movement with relation to the record to receive flux from the record, and means for periodically introducing additional flux into the magnetically permeable 'member when said member receives flux from the record to induce inthe Winding means a periodically varying voltage which is amplitude-modulated in accordance with the recorded signals.

7. in a device for converting a magnetic field recorded on a magnetic record into electrical signals wherein :the magnitude of said signals is made substantially independent of the speed of relative movement between the record and the device, output Winding means, a structure including a portion of variable magnetic permeability for said winding means adapted for relative movement with respect to the record sothat said portion 1" ceives flux from the record, and means for simultaneouslyperiodically introducing additional fiux into said portion to vary periodically the permeability of said portion and develop in said output Winding means a periodically varying voltage which is amplitude-modulated in accordance with the record field.

8. lo apparatus for reproducing signals froma magnetic record, pick-up Winding means, a structure including two magnetically permeable portions for said winding means adapted for relative movement with'relation to the record to receive flux'from the record, an excitation winding for each of said magnetically permeable portions, and means for periodically exciting said last-mentioned winddevelop a periodically varying voltage in said pick-up winding means representative of the recorded signals when said portions receive flux from the record.

9. In a device for reproducing signals from a magnetic record, output Winding means, a structure including two magnetically permeabl portions for said output winding means adapted for relative movement with relation to the record to receive flux from the record, means for periodically substantially saturating said portions when said portions receive flux from the record to unbalance the effective flux in said portions to develop in said output winding means voltages corresponding to the recorded signals.

10. In a device for reproducing signals from a magnetic record, a structure including two magnetically permeable portions adapted for relative movement with respect to the record to receive flux from the record, output winding means magnetically linked with said portions, excitation winding means magnetically linked with said portions, means including said excitation winding means for periodically producing in said portions, in the absence of flux from the record, flux of substantially symmetrical waveform and equal magnitude which periodically increase the saturation in said portions and link said output winding means in an opposite sense to develop zero voltage thereacross and to produce in each of said portions, when flux is received therein from the record, a combined flux of unsymmetrical waveform so that an effective flux corresponding to the diiference between the combined flux in each of said portions is linked to said output winding means to develop therein signals corresponding to t e ec rded s gnals.

11. Apparatus for reproducing signals recorded on a magnetic record wherein the magnitude of the reproduced signals is made substantially independent of the speed of relative movement between the record and the reproducing apparatus, a structure including output Winding means and a portion of variable magnetic permeability for said windin means adapted for relative movement with respect to the record so that said portion receives flux from the record, means including a source of periodically varying electrical energy for simultaneously periodically introducing additional flux into said portion to vary periodically the permeability of said portion and develop in said output winding means a periodically varying voltage Which is amplitude-modulated in accordance with the recorded signals, and means including amplifying means and demodulating means for deriving from said periodically varying amplitude-modulated voltage signals which correspond to the recorded signals.

References Cited in the file of this patent UNITED STATES PATENTS 2,418,553 Irwin Apr. 8, 1947 2,423,339 Newman July 1, 1947 2,456,767 Camras Dec. 21, 1948 2,531,141 Lootens Nov. 21, 1950 2,536,260 Burns Ian. 2, 1951 2,548,109 Howey Apr. 10, 1951 2,608,621 Peterson Aug. 26, 1952 FOREIGN PATENTS 7 2 N r ay Wm-"v". May 2 4 

